1. Field of Invention
This invention relates to ink jet printers.
2. Description of Related Art
A thermal ink jet print head selectively ejects droplets of ink from a plurality of drop emitters to create a desired image on an image receiving member, such as a sheet of paper. The print head typically comprises an array of the drop emitters that convey ink to the image receiving member. In a carriage-type ink jet print head, the print head moves back and forth relative to the image receiving member to print the image in swaths which are many pixels tall.
An ink jet print head typically comprises a plurality of ink passageways, such as capillary channels. Each channel has a nozzle and is connected to an ink supply manifold. Ink from the manifold is retained within each channel until, in response to an appropriate signal applied to a resistive heating element in each channel, the ink and a portion of the channel adjacent to the heating element is rapidly heated and vaporized. Rapid vaporization of some of the ink in the channel creates a bubble that causes a quantity of ink, i.e., an ink droplet, to be ejected from the emitter to the image receiving member. When an ink droplet is ejected at a velocity greater than approximately 5 m/s, the ink droplet is usually accompanied by one or more small sub-drops called satellites.
When a quantity of ink in the form of a ink droplet is ejected from the ejector to a receiving member, the resulting spot and any satellite drops become part of a desired image. Uniformity in spot size of a large number of droplets is very important for image quality. If the volume of droplets ejected from the print head over the course of producing a single document are permitted to vary widely, the lack of uniformity will have noticeable effects on the quality of the image. The most common and important cause of variance in the volume of droplets ejected from the print head is variations in the temperature of the print head over the course of use. The temperature of the liquid ink, before vaporization by the heating element, substantially affects both the bubble growth behavior and the viscosity of the ink. These two properties substantially influence the ejection volume of the ink droplets, and thus the resulting spot size on the image receiving member. The viscosity of the ink also directly affects the resulting spot size, by affecting the spread rate of the droplet after it contacts the image recording member. Control of temperature of the print head has long been of primary concerns in the art.
In order to maintain a constant spot size from an ink jet print head, various strategies have been attempted. One example is U.S. Pat. No. 4,899,180 to Elhatem et al., which is incorporated herein by reference in its entirety. In the'180 patent, the print head has a number of heater resistors and a temperature sensor that operate to heat the print head to an optimum operating temperature and maintain that temperature regardless of local temperature variations.
U.S. Pat. No. 4,791,435 to Smith et al., which is incorporated herein by reference in its entirety, discloses an ink jet system where the temperature of the print head is maintained by using the heating elements of the print head not only for ink ejection but also to maintain the temperature. The print head temperature is compared to thermal models of the print head to provide information for controlling the print head temperature. At low temperature, low energy pulses, i.e., pre-pulses, are sent to each channel, or nozzle, at a voltage that is below the voltage threshold that causes a ink droplet to be ejected. Alternatively, the print head can be warmed by firing some droplets of ink into an external chamber or "spittoon," rather than onto the image receiving member.